1. Field of the Invention
This invention relates to power transmission belts and, more particularly, to a belt which has drive/driven teeth along the length thereof.
2. Background Art
Unlike flat belts, V-belts, and the like, toothed belts are not prone to slipping and therefore are highly efficient in transmitting driving forces. Toothed power transmission belts have an advantage over gears, chains, and the like, because they generally do not generate the same level of noise in operation. Because of these advantages, toothed belts are becoming widely used. As one example, toothed belts are used for synchronous driving of overhead cam (OHC) shafts in automobiles. The field of use for these toothed belts continues to expand.
Toothed belts for driving OHC shafts may be operated in automobile engine compartments at high temperatures, at high speeds, and under heavy loads. Automobiles are being redesigned with higher performance internal combustion engines which are operated in even smaller engine compartments. As a result, toothed belts are required to operate in environments at even higher temperatures and still further to wrap around pulleys with relatively small diameters, which often requires the belts to bend at severe angles.
In known toothed belts, load carrying cords are embedded in a back layer at predetermined intervals along the belt width. When such belts are operated around small diameter pulleys, the load carrying cords must bend sharply and are thus required to be highly flexible. In the absence of the necessary flexibility and resiliency, the load carrying cords may break after a relatively short running period.
In one conventional construction, load carrying cords for toothed belts are prepared by bundling and twisting E-glass/ordinary non-alkali glass fiber filaments. One such load carrying cord construction is shown in JP-A 62-159827.
In one known construction, load carrying cords are prepared by dipping E-glass fiber filaments with a diameter of 9 .mu.m in a resorcinol-formalin resin latex mixture (hereinafter RFL). The filaments are bundled into strands which are twisted in a 3/13 process. That is, 3 strands of RFL-processed glass fiber filaments are primary twisted into a string, followed by final twisting of 13 of the strings into a load carrying cord.
This latter type of belt may have a shortened life in certain high performance internal combustion engines. The life of these high performance internal combustion engines is being increased while the life of the toothed belts used in those engines may not be proportionately lengthened. As a result, the toothed belts used for OHC shaft driving may have to be changed relatively frequently during the life of an automobile.
Another problem with toothed belts in this environment, i.e. at high speeds around small diameter pulleys, is that the fibers making up the load carrying cords may generate a significant amount of frictional heat in the belts. This may degrade the adhesion between a canvas covering cloth and the rubber. This heat may also affect the adhesion between the load carrying cords and the rubber. Further, the heat may have a negative influence on both the canvas covering cloth and the rubber making up the belt body. All of these problems contribute to a potentially shortened life for a toothed belt in this environment.
In JP-B 5-44607U, a toothed belt is described with load carrying cords prepared by processing high-strength E-glass fiber filaments, having a diameter from 6 to 8 .mu.m, with RFL. A predetermined number of the processed filaments are bundled into strands and twisted with a primary twist count of from 7.2 to 8.8 twists/10 cm to produce strings of from 500 to 800 filaments per string. The strings are in turn final twisted at a count of from 7.2 to 8.8 twists/10 cm in a direction opposite to the primary twist direction to produce load carrying cords with 9-12 strings per cord.
Even with this type of load carrying cord, in the environment of the modern internal-combustion engines, the load carrying cords may elongate with the belt run under a heavy load. As a result, the belt itself may be elongated. If this occurs, the belt teeth may not properly engage with cooperating pulleys, eventually potentially leading to abrasion, cracking and potential breakage in a relatively early stage of the anticipated belt life.
Further, if the width of the belt is reduced to minimize the overall size of the belt and associated components, even under an ordinary load, the load per unit width of the belt may be relatively large. The teeth in this situation are likewise prone to cracking and breaking prematurely.
It is known to cover the teeth with a nylon cloth. However, since the initial tension of the belt under heavy load may be high, the abrasion resistance of the nylon cloth may not be satisfactory. The land region between adjacent teeth may become abraded at an early stage in the belt life. The teeth may crack or break to prematurely end the belt life.